[services] Backup - Convert attachment holders from vector to string

Summary:
Depends on D4076

In https://linear.app/comm/issue/ENG-1029/handle-attachments we agreed to keep attachment lists in strings delimited with a single character.

Test Plan: Services still build

Reviewers: palys-swm

Reviewed By: palys-swm

Subscribers: ashoat, palys-swm, Adrian, atul, yayabosh

Differential Revision: https://phabricator.ashoat.com/D4077